1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an engine.
2. Description of the Prior Art
There has been widely employed a so-called lean burn technique in which an amount of air is made excessive (hereinafter, refer to as a lean air-fuel ratio) in comparison with a theoretical air-fuel ratio (about 14.7, hereinafter, refer to as a stoichiometric air-fuel ratio) in order to improve a specific fuel consumption, thereby lean burning the fuel.
For example, there has been going to be spread a structure of realizing a lean burn having an air-fuel ratio of about 20 to 25 of a type injecting a fuel near a suction port of a suction tube portion (a port injection type), and a structure of realizing a significantly lean burn having an air-fuel ratio of about 40 to 50 by forming a stratified mixture of a type directly injecting a fuel into a cylinder (a cylinder internal injection type). In these techniques, it is possible to realize an improvement of a specific fuel consumption by a lean burn, that is, by increasing an amount of the suction air so as to reduce a pumping loss and a heat loss.
However, in the case of the combustion in accordance with the stoichiometric air-fuel ratio, it is possible to simultaneously oxidize and reduce HC, CO and NOx in the exhaust gas by a three way catalysis, however, in the lean burn, since the exhaust gas is under an oxygen excessive state, it is hard to reduce NOx. Accordingly, there has been known an exhaust gas purifying apparatus for an engine structured such that an NOx trapping agent for trapping the NOx in the exhaust gas when the air-fuel ratio of the exhaust gas is lean and discharging the NOx when the air-fuel ratio is rich (the fuel is excessive) is arranged within an exhaust gas passage so as to temporarily change the air-fuel ratio of the exhaust gas from a lean air-fuel ratio to a stoichiometric air-fuel ratio or a rich air-fuel ratio at a predetermined cycle, thereby discharging and reducing the NOx trapped by the NOx trapping agent.
In this kind of exhaust gas purifying apparatus, it is desirable for reducing the specific fuel consumption and a particular component such as an HC or the like in the exhaust gas to make the specific fuel consumption the stoichiometric air-fuel ratio or the rich air-fuel ratio only for a cycle corresponding to an amount of the trapped NOx. There has been suggested Japanese Patent No. 2,692,380 as a technique for judging a completion of discharging the NOx when making the air-fuel ratio the stoichiometric air-fuel ratio or the rich air-fuel ratio. The structure is made such as to judge that the discharge of NOx is completed when the air-fuel ratio detected by an air-fuel ratio sensor mounted downward an NOx absorbent is switched from a lean air-fuel ratio to a rich air-fuel ratio after the air-fuel ratio is switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio. This matter is based on the matter that the HC or the CO in the exhaust gas flowing from an upstream side is consumed for reducing the NOx during the time when the NOx absorbed by the NOx absorbent is discharged and reduced even in the case that the air-fuel ratio upstream the NOx absorbent becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio, so that the air-fuel ratio detected by the air-fuel ratio sensor mounted downstream the NOx absorbent becomes slightly lean and the air-fuel ratio detected by the air-fuel ratio sensor after the discharge and the reduction of the NOx trapped by the NOx absorbent are completed becomes rich.
However, a timing when an output of the air-fuel ratio sensor mounted downstream the NOx absorbent is switched from the lean air-fuel ratio to the rich air-fuel ratio is affected by the air-fuel ratio upstream the NOx absorbent even when the amount of the NOx trapped by the NOx absorbent is uniform. For example, even in the case that the amount of the NOx absorbed by the NOx absorbent is much, the HC or the CO flows inward from the upstream portion of the NOx absorbent over a discharging and reducing speed for the NOx absorbed by the NOx absorbent when making the air-fuel ratio at a time of temporarily changing the air-fuel ratio of the exhaust gas from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio too rich for discharging and reducing the NOx absorbed by the NOx absorbent, so that the HC and the CO which are not consumed flow downstream. Accordingly, the air-fuel ratio detected by the air-fuel ratio sensor mounted downstream the NOx absorbent immediately shows the rich air-fuel ratio, and it is hard to diagnose a performance of the NOx absorbent. In order to prevent such a matter, it is necessary to set the air-fuel ratio upstream the NOx absorbent to a predetermined value.
However, since the air-fuel ratio upstream the NOx absorbent mentioned above is actually a control which is not performed on the basis of a feedback control, the air-fuel ratio is frequently shifted from a target value. Accordingly, it is hard to make the air-fuel ratio at a time of temporarily changing from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio uniform so as to accurately diagnose the performance of the NOx absorbent. Then, at a time of diagnosing an NOx catalyst, it is desirable to employ a method of feedback controlling a control parameter such as the air-fuel ratio or the like at a time of switching the air-fuel ratio from the lean air-fuel ratio to the rich air-fuel ratio so that a time required before the discharge and reduction of the NOx is completed becomes a predetermined value.
Further, in order to quickly perform a diagnosis of the NOx catalyst, it is desirable to employ a method of determining a parameter such as the air-fuel ratio or the like at a time of a rich spike control on the basis of an operating state of the engine so that a threshold used at a time of comparing with a time for completion of the discharge or reduction of the NOx absorbed by the NOx absorbent becomes constant.
In order to solve the problems and achieve the desires mentioned above, in accordance with the present invention, there is provided an engine exhaust gas purifying apparatus comprising:
an NOx trapping agent arranged within an exhaust gas passage, trapping an NOx in an exhaust gas in accordance with an adsorption or an absorption when an air-fuel ratio in the exhaust gas is lean and discharging or reducing the NOx when the air-fuel ratio is a stoichiometric air-fuel ratio or the rich air-fuel ratio;
air-fuel ratio changing means temporarily changing the air-fuel ratio of the exhaust gas from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio at a predetermined cycle;
particular component detecting means detecting a particular component in the exhaust gas downstream the NOx trapping agent;
NOx discharge or reduction completion judging means judging on the basis of a detecting result of the particular component detecting means at a time of temporarily changing the air-fuel ratio of the exhaust gas to the stoichiometric air-fuel ratio or the rich air-fuel ratio by the air-fuel ratio changing means that a discharge or reduction of the NOx trapped by the NOx trapping agent is completed; and
NOx discharge or reduction completion time detecting means detecting a time required for purifying the NOx,
wherein a control parameter of the air-fuel ratio changing means is changed on the basis of a result obtained by comparing a time (TD) detected by the NOx discharge or reduction completion time detecting means with a predetermined value (TTD) previously set.
Further, in accordance with the present invention, there is provided an engine exhaust gas purifying apparatus comprising:
an NOx trapping agent arranged within an exhaust gas passage, trapping an NOx in an exhaust gas in accordance with an adsorption or an absorption when an air-fuel ratio in the exhaust gas is lean and discharging or reducing the NOx when the air-fuel ratio is a stoichiometric air-fuel ratio or the rich air-fuel ratio;
air-fuel ratio changing means temporarily changing the air-fuel ratio of the exhaust gas from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio at a predetermined cycle;
particular component detecting means detecting a particular component in the exhaust gas downstream the NOx trapping agent;
NOx discharge or reduction completion judging means judging on the basis of a detecting result of the particular component detecting means at a time of temporarily changing the air-fuel ratio of the exhaust gas to the stoichiometric air-fuel ratio or the rich air-fuel ratio by the air-fuel ratio changing means that a discharge or reduction of the NOx trapped by the NOx trapping agent is completed; and
NOx discharge or reduction completion time detecting means detecting a time required for purifying the NOx,
wherein the engine exhaust gas purifying apparatus further comprises NOx trapping agent performance judging means for judging a performance of the NOx trapping agent on the basis of a result obtained by comparing a time (TD) detected by the NOx discharge or reduction completion time detecting means with a predetermined value (TTD) previously set, and a control parameter of the air-fuel ratio changing means is previously set on the basis of a parameter indicating an operating state of the engine.
Further, in accordance with the present invention, there is provided an engine exhaust gas purifying apparatus as cited in the structure mentioned above, wherein a difference between the values TD and TTD is made small at a time of changing the control parameter of the air-fuel ratio changing means on the basis of the result of comparison between the detected time TD and the predetermined value TTD.
Further, in accordance with the present invention, there is provided an engine exhaust gas purifying apparatus as cited in the structure mentioned above, wherein the control parameter of the air-fuel ratio changing means is set so that the predetermined value TTD becomes a fixed value without relation to an operating state.
Further, in accordance with the present invention, there is provided an engine exhaust gas purifying apparatus as cited in the structure mentioned above, wherein as the control parameter of the air-fuel ratio changing means, at least one of an air-fuel ratio at a time of temporarily changing to the stoichiometric air-fuel ratio or the rich air-fuel ratio, a cycle for temporarily changing the air-fuel ratio from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio and a term for temporarily changing the air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio is made variable.
Further, in accordance with the present invention, there is provided an engine exhaust gas purifying apparatus as cited in the structure mentioned above, wherein a time that the detected value detected by the particular component detecting means is smaller than the predetermined value is defined as the NOx discharge or reduction completion time.
Further, in accordance with the present invention, there is provided an engine exhaust gas purifying apparatus as cited in the structure mentioned above, wherein a time that the detected value detected by the particular component detecting means is smaller than a first predetermined value and larger than a second predetermined value is defined as the NOx discharge or reduction completion time.
Further, in accordance with the present invention, there is provided an engine exhaust gas purifying apparatus as cited in the structure mentioned above, wherein a deterioration of the NOx trapping agent is detected on the basis of the value of the control parameter of the air-fuel ratio changing means.
Further, in accordance with the present invention, there is provided an engine exhaust gas purifying apparatus as cited in the structure mentioned above, further comprising at least one of storing means for storing a code expressing a deterioration of the NOx trapping agent when a detected deterioration of the NOx trapping agent is equal to or more than a predetermined value, and alarm generating means for generating an alarm.
Further, in accordance with the present invention, there is provided an engine exhaust gas purifying apparatus as cited in the structure mentioned above, wherein a lean operation is limited in correspondence to a detected deterioration of the NOx trapping agent.